There are compositions for dyeing keratinous fiber which perform their function by oxidation of an oxidative color-developing substance, which include, for example, hair dyes and eyebrow dyes. Such compositions for dyeing keratinous fiber conventionally employ hydrogen peroxide as an oxidizing agent. Therefore, most of them are of two-pack type. That is, the oxidative color-developing agent and the oxidizing agent are stored in separate containers, and they are mixed together for reaction at the time of use. The composition of this type is inconvenient to use, and there has been a demand for improvement in their usability. Moreover, it is known that hydrogen peroxide damages keratinous fiber such as hair, and this has produced dissatisfaction with consumers.
One way to address the above-mentioned problem is to replace hydrogen peroxide by an oxidase (as an oxidizing agent) which is previously mixed with an oxidative color-developing substance. The oxidase may be peroxidase (as disclosed in Japanese Patent Laid-open Nos. Sho 47-10400 and Sho 53-32132), laccase (as disclosed in U.S. Pat. No. 3,251,742 and Japanese Patent Laid-open No. Hei 6-172145), or uricase (as disclosed in Japanese Patent Laid-open No. Sho 63-246313).
Among these disclosed technologies, however, the peroxidase-containing composition needs hydrogen peroxide on account of the characteristic properties of peroxidase, and hence the resulting composition cannot be of one-pack type. The uricase-containing composition can be of one-pack type, but it does not solve the basic problem with the use of hydrogen peroxide evolved by the enzyme reaction.
By contrast, in the case where an oxidase which reacts with oxygen as a substrate but does not evolve hydrogen peroxide is used, the composition for dyeing keratinous fiber can be of one-pack type. In addition, such a composition is useful because it does not evolve hydrogen peroxide which damages keratinous fiber (See Japanese Patent Laid-open No. Hei 11-60454). However, it suffers the disadvantage of getting discolored with time during storage, because the enzyme (as a protein) is unstable under the high-temperature high-humidity condition. Therefore, the resulting composition does not permit the enzyme to fully perform its function, and its discoloration is a serious drawback.
It has been known that the above-mentioned discoloration can be effectively eliminated by incorporation with a reducing agent (See “Science of Wave” issued by Shinbiyo Shuppan Co., Ltd.). Improvement of storage stability of enzyme is disclosed in Japanese Patent Laid-open No. Hei 8-175935 (for catalase) and Japanese Patent Laid-open No. Hei 8-217652 (for uricase). These disclosed technologies employ a reducing agent, however, incorporation of a reducing agent into the composition for dyeing keratinous fiber poses a problem. That is, a strong reducing agent lowers the enzyme activity, thereby deteriorating dyeing power, and a weak reducing agent does not fully protect the composition from discoloration.